Internal tourniquet for surgery

ABSTRACT

An internal tourniquet for establishing hemostasis within a portion of a limb to facilitate surgery controls flow of a fluid into a capsule surrounding substantially all of a human joint. Blood concentration in the capsule is sensed; and pressure in the capsule is controlled to maintain a fluid pressure in the capsule within a predetermined pressure tolerance window. The concentration of blood in the capsule is maintained below a predetermined maximum concentration while the fluid pressure is within the pressure tolerance window.

FIELD OF THE INVENTION

This invention pertains to tourniquets used for stopping blood flow in aportion of a surgical patient's limb to facilitate the performance of asurgical procedure.

BACKGROUND OF THE INVENTION

External tourniquet systems have long been used to establish hemostasisin the upper and lower limb in order to facilitate orthopedic surgicalprocedures. External tourniquet systems of the prior art typicallyinclude a pneumatic tourniquet cuff applied around a patient's limbproximal to a desired surgical field, and an external tourniquetinstrument for supplying the cuff with gas at a pressure above theminimum pressure needed to stop arterial bloodflow past the cuff andinto the surgical field for the duration of a surgical procedure. Inthis way, an external tourniquet system establishes a bloodless andclear surgical field in the limb distal to the cuff, allowing complexorthopedic surgical procedures to be performed with improved accuracy,safety and speed. Many types of external tourniquet systems have beendescribed in the prior art, such as those described by McEwen in U.S.Pat. No. 4,469,099, No. 4,479,494, No. 5,439,477 and McEwen and Jamesonin U.S. Pat. No. 5,556,415 and No. 5,855,589. However, in addition toestablishing a bloodless surgical field, external tourniquet systems ofthe prior art also stop bloodflow to non-surgical regions of the limb,resulting in ischemia and a risk of injury to these non-surgical regionsthat increases as the duration of ischemia increases. Further, externaltourniquet systems of the prior art apply pressure to underlying muscle,blood vessels and nerves proximal to the surgical site, resulting in arisk of injury to these tissues that increases as the level of pressureand duration of pressure application increases. In addition, anatomicalconsiderations in certain surgical procedures, such as in hip andshoulder surgeries, may limit or completely prevent the use of externaltourniquet systems for establishing hemostasis.

If an external tourniquet system is not used, then other apparatus knownin the prior art may be employed to improve visualization and reducebleeding, especially for arthroscopic surgical procedures. Someprior-art apparatus manage the flow of sterile fluid into and out of acapsule that envelops a joint, thereby to help establish a pressurewithin the capsule that may reduce bleeding and improve visualization,and to help remove surgical debris and blood that may be present in thecapsule. Prior-art fluid management systems for arthroscopic surgery aredescribed, for example, by Chandler et al. in U.S. Pat. No. 5,800,383,by Beiser et al. in U.S. Pat. No. 5,840,060 and U.S. Pat. No. 5,662,611.Arthroscopic fluid management systems known in the prior art requiresignificant manual intervention, knowledge, skill and attention by thesurgeon throughout a surgical procedure in order to balance a number ofcompeting requirements: the control of bleeding in the surgical field,the minimization of extravasation or swelling of tissues surrounding thesurgical field and its associated risk of patient injury; the removal ofblood and surgical debris from the surgical field; the maintenance ofacceptable visualization; and the minimization of fluid loss and itsrelated costs and hazards.

To facilitate new orthopedic surgical procedures that are becoming lessinvasive, as well as to facilitate arthroscopic surgical procedures thatare becoming increasingly complex, there is a need for an internaltourniquet that can establish adequate hemostasis and provide asufficiently bloodless surgical field over a time period suitably longfor the performance of a surgical procedure without the limitations inperformance and without the risks of patient injuries associated withprior-art external tourniquet systems and arthroscopic fluid managementsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one preferred embodiment in an orthopedicsurgical application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A block diagram of a preferred embodiment of the invention in use duringan orthopedic surgical procedure is depicted in FIG. 1. Internaltourniquet 2 supplies clear pressurized fluid to joint capsule 4 topermit a surgical procedure to be performed within the region 6 enclosedby the joint capsule 4. Joint capsule 4 is a substantially fluid-tightsac-like envelope that encloses the cavity of a synovial joint byattaching to the circumference of the articular end of each involvedbone 8 and 10.

To visualize the bones and tissues within the joint the operatingsurgeon inserts a scope 12 into the joint capsule 4. Scope 12 includes acannulated sheath that provides a fluid passageway from internaltourniquet 2 to the interior region 6 of joint capsule 4, as shown inFIG. 1. Scope 12 is typical of commonly used scopes and contains fiberoptic fibers coupled to a light source for transmitting light into jointcapsule 4 and magnifying lenses coupled to a color video camera forvisualizing the interior of region 6 of joint capsule 4. Video signalsfrom the camera of scope 12 are transmitted to video display 14 forviewing by the operating surgeon and to internal tourniquet 2 foranalysis as described below. Pressurized fluid from internal tourniquet2 is supplied to scope 12 via flexible plastic tubing 16.

Surgical instruments may be inserted into the joint capsule 4 viainstrument portal 18. Instrument portal 18 forms a substantiallyfluid-tight seal around the body of a surgical instrument as it isinserted through the portal, the fluid-tight seal of instrument portal18 may be adapted to accommodate surgical instruments of variousdiameters and shapes. Instrument portal 18 includes retaining elements20 as shown in FIG. 1. Retaining elements 20 form a fluid-tight sealbetween the body of instrument portal 18 and joint capsule 4, therebypreventing fluid loss from the capsule and extravasation of fluid intosurrounding tissues. Retaining elements 20 also provide robust fixationof the portal for the duration of the surgical procedure and improveretention of the portal while surgical staff insert, manipulate andremove surgical instruments.

During the time period of a surgical procedure bleeding may occur fromtissues and bones within region 6 of joint capsule 4 and from thetissues surrounding joint capsule 4 where openings into the capsule havebeen made. The presence of blood within the region can impede theability of the operating surgeon to clearly visualize both the interiorof the joint and any surgical instruments that may be present within theregion; this may result in unnecessary delays, an increase in the riskof patient injury and a decrease in the precision of the surgicalprocedure.

Internal tourniquet 2 acts to help improve visualization of the interiorof joint capsule 4 by maintaining hemostasis within region 6 of jointcapsule 4. Internal tourniquet 2 supplies pressurized fluid to the jointcapsule via scope 12, and controls the pressure of the fluid within thejoint capsule and rate of fluid flow through the joint capsule. Asdescribed further below, internal tourniquet 2 maintains hemostasiswithin joint capsule 4 by actively adjusting the fluid pressure and flowwithin the joint capsule in response to changes in the amount of blooddetected within the fluid both within the joint capsule and exiting thejoint capsule. To reduce extravasation, internal tourniquet 2 acts tomaintain the fluid pressure near the lowest pressure necessary tomaintain hemostasis over a time period suitably long for the performanceof a surgical procedure.

As shown in FIG. 1, internal tourniquet 2 consists of the followingfunctional modules: user interface and display 22, flow and pressurecontrol module 24, hemostasis control module 26, blood detection module28 and video blood sensor 30. Internal tourniquet 2 is supplied withfluid from fluid reservoir 32 via flexible plastic tubing 34. Fluidreservoir 32 contains sterile normal saline solution, lactated ringerssolution or other clear and sterile fluid suitable for thepressurization of joint capsule 4.

User interface and display 22 includes a LCD panel for the display of:pressure values, flow rate values, sensed blood concentration values andalarm conditions. Hemostasis control parameters are also displayed;these include minimum and maximum pressure limits, minimum and maximumflow limits and the operating modes of hemostasis control module 26. Amembrane key pad forms part of user interface and display 22 for theadjustment of hemostasis control parameters and operating modes. Userinterface and display 22 also includes an audio transducer for alertingthe operator to alarm conditions.

Flow and pressure control module 24 within internal tourniquet 2includes a pump that pressurizes fluid from fluid reservoir 32; thepressurized fluid is supplied to joint capsule 4 via tubing 16 and scope12. Flow and pressure control module 24 responds to a fluid flow ratereference signal and a fluid pressure reference signal from hemostasiscontrol module 26. Flow and pressure control module 24 acts to maintainthe pressure within joint capsule 4 near the pressure level set by thefluid pressure reference signal and acts to maintain the fluid flow rateto joint capsule 4 near the rate set by the fluid flow rate referencesignal.

Pressure transducer 36 generates a pressure signal. The level of thepressure signal is representative of the pressure of fluid sensed withinregion 6 of joint capsule 4. The pressure signal is communicated to flowand pressure control module 24. For clarity in FIG. 1, pressuretransducer 36 is shown as a separate device extending into joint capsule4; it will be apparent that pressure transducer 36 could be combinedwith scope 12, instrument portal 18 or other apparatus in directcommunication with the fluid-filled region 6 of joint capsule 4.

The fluid supplied to joint capsule 4 by flow and pressure controlmodule 24 is removed from joint capsule 4 through fluid outlet portal 38and flexible plastic tubing 40. An outlet control valve 42 acts upontubing 40 to control the rate at which fluid leaves joint capsule 4. Inthe preferred embodiment outlet control valve 42 is a servo controlledpinch valve which acts upon tubing 40 in response to an outlet controlsignal from flow and pressure control module 24. Fluid supplied to jointcapsule 4 may also leave the joint capsule through extravasation,through instrument portal 18, and through surgical incisions extendinginto joint capsule 4.

By varying the degree of restriction in tubing 40 and the rate at whichfluid is supplied to joint capsule 4, flow and pressure control module24 acts to maintain the pressure level within joint capsule 4 and thefluid flow rate through joint capsule 4 near the levels set byhemostasis control module 26. In the preferred embodiment, fluidpressure and flow within joint capsule 4 are maintained near set levelswith a controllable fluid pump and a variable outlet restriction, itwill be apparent that other means may be used to maintain fluid flowthrough the capsule and pressure within the capsule near desired levels.For example, two pumps may be used, one supplying fluid to the capsulethe other acting as a variable restriction or suction source to removefluid from the capsule; as another example, fluid from a pressurizedsource could be supplied to the capsule through a controlled restrictingvalve and removed from the capsule via a controlled restricting valve orpump.

Flow and pressure control module 24 communicates the pressure signal anda flow rate signal representative of the rate of fluid flowing into thejoint capsule 4 to hemostasis control module 26 and to user interfaceand display 22 for display to the user. The flow rate signal may beobtained by any suitable flow sensor or meter associated with the flowand pressure control module 24.

Blood detection module 28 receives and processes blood concentrationsignals from blood sensors 44 and 46 and video blood sensor 30. Bloodsensors 44 and 46 are optical calorimetric sensors sensitive to theabsorption spectra of hemoglobin, a hemoprotein composed of globin andheme that gives red blood cells their characteristic red color;. Bloodsensors 44 and 46 transmit light with predetermined wavelengths througha volume of fluid to a photodetector to produce signals indicative ofthe light absorption, hence the amount of hemoglobin detected within thefluid, which is representative of the concentration of blood presentwithin the fluid. Blood sensor 44 (BLD-6.0, Edgewood, N.Y.) is shown inFIG. 1 applied to tubing 40 that is external to the joint capsule 4.This sensor detects the amount of hemoglobin present in a predeterminedsample volume of fluid exiting the joint capsule through outlet portal38 and tubing 40. Blood sensor 46 operates on similar principles tosensor 44 and is inserted directly into the joint capsule. It alsoproduces a signal indicative of the amount of hemoglobin present withina predetermined sample volume of fluid within joint capsule 4. Forclarity in FIG. 1, blood sensor 46 is shown as a separate deviceextending into joint capsule 4; it will be apparent that blood sensor 46could form part of scope 12, instrument portal 18 or other apparatus indirect communication with the fluid-filled region 6 of joint capsule 4.

Video blood sensor 30 receives video signals from the color video camerathat forms part of scope 12 and analyses these video signals todetermine the concentration of blood present in the fluid within thevisual field of the scope. Video blood sensor 30 produces a video bloodconcentration signal indicative of the amount of blood sensed within thevisual field of the scope that is communicated to blood detection module28. Video blood detector contains a computer processor with softwarealgorithms that digitize and analyze in real time the video signals fromthe video camera of scope 12. The video analysis algorithms identify redcolored regions of the visual field. The probability that a regioncontains a concentration of blood is computed based on: the detectedfeatures present within the region, a comparison with surroundingregions; and a comparison with regions in previous video frames.Identified regions with a computed probability that exceed apredetermined minimum probability threshold are then quantified todetermine the level of the video blood concentration signal communicatedto blood detection module 28.

Blood detection module 28 receives inputs from blood sensors 44, 46 andvideo blood sensor 20. Blood detection module contains algorithms toprioritize, compare and assign weighting values to the bloodconcentration signals from the sensors and produces a detected bloodconcentration signal; the level of this signal is indicative of theamount of blood present within joint capsule 4. This detected bloodconcentration signal is communicated to hemostasis control module 26 anduser interface and display 22. Although three different blood sensorsare shown and described in the preferred embodiment it will be apparentthat blood detection module 28 may be adapted to produce a detectedblood concentration signal from a lesser or greater number of bloodsensors and may be adapted to accept signals from blood sensors based onother measurement principles.

The hemostasis control module 26 of internal tourniquet 2 receives thedetected blood concentration signal from blood detection module 28 andreceives hemostasis control parameters from user interface and display22. Hemostasis control module 26 also receives the pressure and flowrate signals from flow and pressure control module 24. Hemostasiscontrol module 26 produces the fluid pressure reference signal and thefluid flow rate reference signal which are communicated to flow andpressure control module 24. As described above, the levels of thesesignals control the fluid pressure and the flow rate that fluid controlmodule 24 will maintain in region 6 of joint capsule 4.

Hemostasis control module 26 acts to maintain a bloodless surgical fieldby automatically adjusting the levels of the fluid pressure and fluidflow rate reference signals in response to changes in the level of thedetected blood concentration signal.

Hemostasis control module 26 adjusts the level of the fluid pressurereference signal so that the pressure signal is maintained at a levelbetween the minimum pressure limit and the maximum pressure limit.Hemostasis control module 26 also adjusts the level of the fluid flowrate reference signal so that the flow rate signal is maintained at alevel between the minimum flow rate limit and the maximum flow ratelimit. The values of the minimum and maximum pressure and flow limitsmay be set individually by an operator of internal tourniquet 2 via userinterface and display 22 or be set automatically to predetermined valuesby hemostasis control module 26.

If for any reason, hemostasis control module 26 and fluid and pressurecontrol module 24 cannot maintain the pressure signal at a level that iswithin the pressure tolerance window formed by the minimum and maximumpressure limits an alarm signal is generated when the pressure signal isoutside the tolerance window. For example if the minimum pressure limitis set at 10 mmHg and the maximum pressure limit is set at 50 mmHg, thealarm signal will be generated if the fluid pressure is less that 10mmHg or greater than 50 mmHg. The alarm condition is indicated to theoperator by user interface and display 22. Similarly, if the level ofthe flow rate signal cannot be maintained within the flow tolerancewindow formed by the minimum and maximum flow rate limits an alarmsignal is also generated to alert the user that that internal tourniquet2 cannot maintain the desired pressures or flows. Examples of someconditions that may cause alarms are: the occlusion of outlet portal 38or tubing 40 by surgical debris; substantial fluid leaks from jointcapsule 4; occlusion of tubing 16 or the fluid pathway within scope 12,and failure of capsule pressure sensor 36.

Hemostasis control module 26 has two operating modes: pressure-preferredmode and flow-preferred mode. The operating mode of hemostasis controlmodule 26 may be set by the operator via user interface and display 22or may be set automatically by hemostasis control module 26 in responseto predetermined fluid pressure levels, fluid flow rates and regionblood concentrations.

When operating in pressure-preferred mode, hemostasis control module 26maintains the fluid pressure reference signal at a constant level andresponds to an increase in the amount of blood present within region 6,as indicated by the level of the detected blood concentration signal, byproportionally increasing the level of the fluid flow rate referencesignal. This causes an increase in fluid flow through joint capsule 4which acts to clear blood from the capsule and restore a clear operatingfield. When the concentration of blood detected in the capsuledecreases, the fluid flow rate reference signal is also decreased byhemostasis control module 26 until the level of the flow rate signal isnear the minimum flow rate limit, this acts to conserve the fluid influid reservoir 32. When adjusting the fluid flow rate reference signal,hemostasis control module 26 maintains the level of the flow rate signalwithin the minimum and maximum fluid flow rate limits. If, whileadjusting the fluid flow rate reference signal to control bleedingwithin the region 6, hemostasis control module 26 increases the fluidflow rate reference level to the maximum flow rate limit and the levelof the detected blood concentration signal exceeds a predeterminedminimum level, an alarm signal is generated to indicate to surgicalstaff that internal tourniquet 2 cannot adequately control the bleedingwithin the region 6.

When operating in flow-preferred mode, hemostasis control module 26maintains the fluid flow rate reference signal at a constant level andresponds to an increase in the amount of blood present within region 6,as indicated by the level of the detected blood concentration signal, byincreasing the level of the fluid pressure reference signal. This causesan increase in fluid pressure within joint capsule 4 which acts toprevent blood from entering the capsule and obscuring the operatingfield. When adjusting the fluid pressure reference signal hemostasiscontrol module 26 maintains the level of the fluid pressure signalwithin the minimum and maximum fluid pressure limits. If while adjustingthe fluid pressure reference signal to control bleeding within theregion 6, hemostasis control module 26 increases the fluid pressurereference level to the maximum pressure limit and the level of thedetected blood concentration signal exceeds a predetermined minimumlevel an alarm signal is generated to indicate to a user that internaltourniquet 2 cannot control the bleeding within the region 6.

To continuously maintain a bloodless surgical field for the duration ofa surgical procedure, hemostasis control module 26 may be adapted toautomatically change operating modes and vary the minimum pressure andflow limits, in response to changes in the level of the detected bloodconcentration signal and pressure and flow rate signals.

In FIG. 1 an external non-invasive blood pressure (NIBP) monitor 48 isshown in communication with hemostasis control module 26. NIBP monitor48 is an external device that non-invasively measures the blood pressure(BP) of the surgical patient. For example, the NIBP monitor may measureBP intermittently using an oscillometric technique, or may measure BPcontinuously using pulse wave transit time. NIBP monitor 48 produces aBP signal indicative of the value of the patient's systolic bloodpressure which is communicated to hemostasis control module 26. Inresponse to hemostasis control parameters set by an operator via userinterface and display 22, hemostasis control module 26 may operate tomake adjustments in the level of the fluid pressure reference signal inresponse to changes in the patient's blood pressure. For example, if thepatient's blood pressure rises hemostasis control module 26 can increasethe fluid pressure within the window formed by the minimum and maximumfluid pressure limits. Similarly, if the patient's blood pressuredecreases hemostasis control module 26 can decrease the fluid pressure.By acting in response to changes in the BP signal to adjust the fluidpressure within region 6, hemostasis control module 26 can bettermaintain a bloodless surgical field. It will be apparent that otherdevices and methods could be used to determine the systolic bloodpressure and that hemostasis control module 26 could similarly respondto systolic blood pressure signals produced by other external devices.It will also be apparent that an NIBP monitor could be incorporatedwithin internal tourniquet 2 to eliminate the need for a separatedevice.

For clarity the preferred embodiment described above has been showncontrolling hemostasis within the region enclosed by the substantiallyfluid-tight capsule of an articulating joint. The embodiment describedmay be adapted to help control hemostasis during other minimallyinvasive surgical procedures within a partially open or open region neara joint or bone where a bloodless surgical field must be maintained.

The embodiment illustrated is not intended to be exhaustive or limit theinvention to the precise form disclosed. It is chosen and described inorder to explain the principles of the invention and its application andpractical use, and thereby enable others skilled in the art to utilizethe invention.

1. An internal tourniquet for establishing hemostasis within a portionof a limb to facilitate surgery comprising: fluid delivery means adaptedto facilitate flow of a fluid into a capsule surrounding the limbportion; blood sensing means for sensing a concentration of blood in thefluid within the capsule; and pressure regulation means responsive tothe sensed blood concentration to control the fluid delivery means toregulate the concentration of blood in the fluid in the capsule, and tomaintain fluid pressure in the capsule within a pressure tolerancewindow.
 2. The apparatus as described in claim 1 wherein the fluiddelivery means controls fluid outflow from the capsule to maintain theconcentration of blood in the capsule below a predetermined maximum. 3.The apparatus as described in claim 1 wherein the pressure regulationmeans includes pressure sensing means for sensing a level of fluidpressure in the capsule, wherein the pressure regulation means isresponsive to the pressure sensing means for controlling the fluiddelivery means to maintain the fluid pressure in the capsule within thepredetermined pressure tolerance window.
 4. The apparatus as describedin claim 1 wherein the blood sensing means produces an indication of theamount of hemoglobin present in the fluid in the capsule.
 5. Theapparatus as described in claim 1 wherein the fluid flowing to thecapsule is substantially transparent and wherein the blood sensing meansproduces an indication of the amount of hemoglobin present in the fluidin the capsule by measuring absorption of light that is directed througha predetermined volume of the fluid.
 6. The apparatus as described inclaim 1 including blood alarm means for producing a blood alarm signalif the sensed concentration of blood is greater than a predeterminedmaximum concentration.
 7. An internal tourniquet comprising: fluiddelivery means adapted to facilitate flow of a fluid into a region neara human joint; blood sensing means for sensing a concentration of bloodin the fluid in the region and for producing a blood signal indicativeof the concentration; and pressure regulation means adapted forcontrolling the fluid delivery means to maintain a fluid pressure in theregion within a pressure tolerance window over a time period suitablylong for the performance of a surgical procedure, wherein the pressureregulation means is further responsive to the blood signal for furthercontrolling the fluid delivery means to maintain the concentration ofblood below a predetermined maximum concentration when the fluidpressure is within the pressure tolerance window.
 8. The apparatus asdescribed in claim 7 and including fluid outflow means adapted tofacilitate outflow of the fluid from the region, wherein the pressureregulation means is further adapted to further control the fluid outflowmeans to maintain the concentration of blood below the predeterminedmaximum when the fluid pressure is within the pressure tolerance window.9. The apparatus as described in claim 7 and including pressure sensingmeans for sensing a level of fluid pressure in the region and forproducing a pressure signal indicative of the level of fluid pressure,wherein the pressure regulation means is further responsive to thepressure signal for controlling the fluid delivery means to maintain thefluid pressure in the region within the predetermined pressure tolerancewindow.
 10. The apparatus as described in claim 9 and including pressurealarm means responsive to the pressure signal for producing a pressurealarm signal if the level of fluid pressure indicated by the pressuresignal is outside the predetermined pressure tolerance window.
 11. Theapparatus as described in claim 7 and including blood alarm means forproducing a blood alarm signal if the concentration of blood indicatedby the blood signal is greater than the predetermined maximumconcentration.
 12. An internal tourniquet comprising: fluid inflow meansadapted to facilitate flow of a fluid into a surgical region near ahuman joint; blood sensing means for sensing a concentration of blood inthe fluid and for producing a blood signal indicative of theconcentration; and fluid flow control means responsive to the bloodsignal and adapted for controlling the fluid inflow means to establish arate of flow of the fluid to the region sufficient for maintaining theconcentration of blood indicated by the blood signal below apredetermined maximum concentration over a time period suitably long forthe performance of a surgical procedure.
 13. The apparatus as describedin claim 12 and including fluid outflow means providing a substantiallyfluid-tight conduit for the fluid to flow out of the region, wherein thefluid flow control means is further adapted for controlling the fluidoutflow means to establish the rate of flow.
 14. The apparatus asdescribed in claim 12 wherein the blood sensing means produces anindication of the amount of hemoglobin present in the fluid in theregion.
 15. The apparatus as described in claim 12 wherein the fluidflowing into the surgical region is substantially transparent andwherein the blood sensing means produces an indication of the amount ofhemoglobin present in the fluid by measuring the absorption of lightdirected through a predetermined volume of the fluid.
 16. The apparatusas described in claim 12 wherein the blood sensing means produces anindication of the amount of hemoglobin present in the fluid by measuringthe absorption of light of various wavelengths.
 17. The apparatus asdescribed in claim 13 wherein the blood sensing means is adapted tosense the concentration of blood by measuring the amount of hemoglobinpresent in the fluid flowing out of the region though the substantiallyfluid-tight conduit of the fluid outflow means.
 18. A method ofoperating an internal tourniquet that includes a capsule through whichfluid may be directed, comprising the steps of: adjusting the fluid flowrate through the capsule in response to changes in the amount of bloodin the capsule.
 19. The method of claim 18 also including the step ofadjusting the level of fluid pressure in the capsule in response tochanges in the amount of blood in the capsule.
 20. The method of claim17 including the step of monitoring the pressure in the capsule andmaintaining the fluid pressure therein within a pressure tolerancewindow.